1. Field of the Invention
The present invention relates to a die, a method of manufacturing a stepped metal pipe or tube, and a stepped metal pipe or tube. The invention more specifically relates to a die for use in an extrusion process for reducing the diameter of a metal pipe or tube, a method of manufacturing a stepped metal pipe or tube using the die, and a stepped metal pipe or tube.
2. Description of the Related Art
Among automobile parts such as a shaft, some parts have a stepped shape of varying diameter in the axial direction (hereinafter referred to as “stepped parts”) as shown in FIG. 1. Such a stepped part is manufactured by subjecting a solid material to an extrusion process and reducing its diameter. Referring to FIGS. 2A to 2D, a columnar solid material is cut into billets 1 having a prescribed length (FIG. 2A). Then, a billet 1 is placed in the vertical direction on a die 2 for extrusion, and a press 3 is placed on the upper end of the billet 1 (FIG. 2B). The billet 1 is then pushed into a through hole 21 of the die 2 and the lower end of the billet 1 is forced out from the lower surface of the die 2 (FIG. 2C). The lower end of the billet 1 is extruded to protrude a prescribed distance from the lower surface of the die 2, and then the billet 1 is pushed out from the die 2 using a push-out jig 4 (FIG. 2D). By these processes, the billet 1 is formed into a stepped part.
As shown in FIG. 2B, the through hole 21 of the die 2 has an inside surface including a bell portion 211, an approach portion 212, a bearing portion 213, and a relief portion 214 formed in a continuous manner. The bell portion 211 serves to guide the billet 1 toward the approach portion 212. Compressing force in the radial direction is exerted for the first time on the billet 1 by the approach portion 212, and the diameter of the billet is reduced. The die half angle R1 of the approach portion 212 is usually fixed.
In recent years, in order to manufacture more lightweight automobiles, stepped metal pipes or tubes produced by extruding hollow metal pipes or tubes are coming to be used as stepped parts.
However, when a stepped metal pipe or tube is produced by a conventional extrusion process using the die 2, the cylindrical portion with a reduced diameter is bent as shown in FIG. 3. A stepped metal pipe or tube attached to an automobile usually rotates in the axial direction. A bent stepped metal pipe or tube is not preferable because it vibrations during rotation.
Japanese Patent Laid-Open No. 2002-11518 discloses a die for use in a drawing process. Unlike the extrusion process carried out without fixing the tip end of the material, the tip end of the material is chucked while it is pulled out in the drawing process, and therefore it is not easy for bending to occur. Therefore, the die for drawing and the die for extrusion have different shapes.